Radiation curing has become more and more a technology of choice for several reasons. Radiation curing provides low or zero emission of volatile organic compounds (VOCs), is energy efficient, does not require combustion of fossil fuels, which produces resultant carbon dioxide emissions and provides high productivity. Radiation cure technology is widely used in coatings, inks and adhesives. The curable compositions may include mixtures of monomers, oligomers, photoinitiators and additives which are applied to a substrate and cured in place via exposure to ultraviolet (UV) light. The adhesion of the cured compositions to substrates varies from system to system. Various additives have been used to improve adhesion. For example, silane agents have been widely used to improve adhesion in traditional coatings and composites based on epoxy chemistry. However, the application and selection of silane agents depends on cure mechanisms. In the case of radiation curable compositions, only one compound, gamma-methacryloxypropyl trimethoxysilane, is compatible with the cure mechanism and is readily available. Other silane-based coupling agents are available, but are mainly directed to two-part, non-(meth)acrylate systems. See Waldman, Silane Coupling Agents Improve Performance, Modern Paints and Coatings, February, 1996.
Gamma-methacryloxypropyl trimethoxysilane monomer is a commercially available coupling agent for bonding coatings to substrates. The coupling agent may be mixed with other copolymerizable monomers, such as (meth)acrylates (e.g., acrylates, methacrylates or mixtures thereof) and the mixture of monomers may be applied to a surface and cured. The methacryloxypropyl trimethoxysilane coupling agent of the prior art is prepared from allyl methacrylate, a volatile, odorous compound which is also a strong skin and eye irritant. The presence of the allyl methacrylate starting material along with the methacryloxypropyl trimethoxysilane monomer coupling agent also causes odor problems with the cured coatings.
The methacryloxypropyl trimethoxysilane coupling agent of the prior art is also a mono-methacrylate and is extractible from coatings when not fully polymerized and crosslinked into the cured network. In certain applications, this extractible monomer may be considered as an undesirable contaminant. The adhesion benefits obtained from this monomer are also limited because it only contains a single trimethoxy group to promote adhesion or coupling to substrate or fillers.
U.S. Pat. No. 6,391,463 is directed to alkoxylated modification of a methacryloxypropyl trimethoxysilane coupling agent as a means to overcome odor and skin and eye irritancy. It is also based on an easier to handle alkoxylated allyl methacrylate precursor. This improvement is limited, however, as the disclosed compositions still only contain a single methacrylate function and a single trimethoxysilane adhesion promoting functional group. Additionally, the composition is based on alkoxylate allyl methacrylate, which is not commercially available.
WO 96/12749 is directed to silane oligomers and radiation curable coating compositions for optical fiber coating. The silane oligomers are high in molecular weight (e.g., 500-11,000) and a high level of silane oligomer is needed, typically about 5-99% of the coating composition. These silane oligomers are typically prepared based on urethane chemistry and urethane linkage.
The abstract for DE-4416857 relates to hydrolysable and polymerizable silanes, which have carboxylic acid functional groups, for use in free radical polymerization. The silane agents are prepared from hydroxy containing compounds and acid anhydride containing silanes.
Additionally, there is developing art where curable silanes may be used with other cure technologies to make harder, more durable “nanocomposite” type coatings and adhesives. For example, university researchers are investigating reinforcement of UV cured coatings by incorporating alkoxy silane/(meth)acrylate functional components in formulas with both free radical and cationic generating photoinitiators thereby causing formation of sol-gel networks within acrylic polymer networks on exposure to UV irradiation. See Journal of Polymer Science, Part A, Polymer Chemistry, Vol 48, 4150-4158 (2010) and Macromolecular Materials and Engineering, 2011, vol. 296, issue 6, pp. 506-516.
There remains a need, however, for cure technologies having reduced volatility and odor while still providing effective adhesion performance, hardness, etc.